Plasma processes are widely used in semiconductor manufacturing, for example, to implant wafers with various dopants, or to deposit or to etch thin films. During a plasma ion implantation process such as plasma doping processes (PLAD), ions are extracted through a high voltage sheath and accelerated toward a target.
In a typical PLAD process, a plasma may be monitored by measuring an implant dose based on a Faraday cup current. However, a Faraday cup is limited to measuring total charge, which does not distinguish between different charged particles or otherwise offer any insight into the plasma. In addition, in pulsed plasma processing wherein a plasma alternates between on and off states, time-resolved measurements of the plasma are often required.
To address some of the above concerns, a time-of-flight ion sensor technique for monitoring ions in a plasma has been developed. In particular, U.S. Pat. No. 7,476,849, issued Jan. 13, 2009, and incorporated by reference herein in its entirety, discloses the use of a time-of-flight sensor to monitor at a fixed angle the plasma species including ions that may impinge on a substrate.
Another method of implanting wafers with various dopants is through the use of ion implantation using a beamline tool. For example, in conventional beamline tools (as well as newly developed ion beam systems based upon plasmas) ions may impinge on a workpiece over a wide range of angles. FIG. 1 is a cross-sectional view of a known focusing plate arrangement for implanting ions having a wide angular distribution on a substrate. The focusing plate 11 is configured to modify an electric field within the plasma sheath 242 to control a shape of a boundary 241 between plasma 14 and the plasma sheath 242. Accordingly, ions 102 that are attracted from the plasma 14 across the plasma sheath 242 may implant into the workpiece 10 at a large range of incident angles (see, e.g., ion trajectories 269-271).
In addition to sampling ion species, it may be desirable to measure the angular distribution of ions in such ion beam systems. For example, the exact distribution of ion angles, energies, and mass may be critical in determining the resulting properties of devices formed by such a process. However, none of the aforementioned techniques provide information regarding the mass, ion energy, and angular distribution of ion species.
In view of the above, it will be apparent that in order to achieve predictable and repeatable process results for ion implantation, there is a need to closely monitor and control ion parameters, such as ion energy, angular distribution, and mass.